A solar cell power generation system having numerous solar cell panels arranged therein is getting into widespread use in order to effectively utilize solar energy. Such a solar cell power generation system has various kinds such as a small-sized system which is installed on a roof of a house and a large-scaled system which has generation power of mega watt or higher and can provide electric power throughout a region.
As illustrated in FIG. 1, in a typical solar cell power generation system, a solar cell string 101 is configured by connecting in series solar cell modules 111, 112, and 113 combined with bypass diodes 104, respectively, and further, backflow preventing diodes 141 and 142 are connected to the power output terminal of the solar cell string 101. The solar cell string 101 is connected at both ends thereof to power cables 150 and 151, respectively, so as to collect electric power. There are provided numerous solar cell strings 101, each having the above-described configuration. Electric power generated in each of the solar cell strings 101 is collected via the power cables 150 and 151, is sent to a single power collecting terminal apparatus, and thus, is used as an output from the solar cell power generation system.
The bypass diode 104 has the function of, when electromotive force of a certain one of the solar cell modules is degraded, bypassing a current generated in the other solar cell module. Each of the backflow preventing diodes 141 and 142 has a function of preventing any backflow of a current to the solar cell string having a lower potential in the case where a difference in potential is generated between the solar cell strings. In most cases, the backflow preventing diodes 141 and 142 are two diodes connected to each other in series for withstanding a high voltage.
In general, abnormality in the solar cell power generation system has been conventionally detected per solar cell module constituting a solar cell panel or detected per solar cell string consisting of a plurality of solar cell modules.
For example, Patent Document 1 discloses a solar cell module including detecting means for detecting a current or a voltage per solar cell module and communication means for performing communication according to an output from the detecting means, so as to easily specify the occurrence of a failure in the solar cell module and a troubled solar cell module.
Alternatively, Patent Document 2 discloses a characteristic evaluating apparatus for a solar cell, including a measuring unit for measuring current-voltage characteristics in units of a plurality of solar cell modules, a converting unit for converting the measured current-voltage characteristics into a predetermined standard, a memory storing a plurality of standard characteristics therein, and a determining unit for determining by comparing the current-voltage characteristics converted into the standard status and each of the standard characteristics read from the memory.
In the above-described detecting apparatus or evaluating apparatus, in general, a power source for the apparatus is provided between the solar cell modules to be measured or a battery is independently provided for supplying a power to the apparatus. In the former case, a circuit is required for decreasing the obtained high voltage down to a proper use voltage in the order of about 1/100, thereby raising a problem of consumption of a part of electric power generated in the solar cell. In contrast, in the latter case, there arises the problem of control at the time of exhaustion or replacement of a cell, thereby making the apparatus complicated as a whole. Not only the abnormality detecting unit in the solar cell power generation system but also transmission means for the detected data are complicated. Moreover, accurate abnormality detection by a simple apparatus of a low cost has been required for a mega solar system using many solar cell panels.